Add requirement rh-java-common-google-gson to the RPM spec file [1].

[1] https://github.com/jbdevstudio/jbdevstudio-product/blob/master/rpm/devstudio.spec.template#L48

Reason:

After update form Devstudio 10.2 to 10.4 I had this error in the eclipse log:

!ENTRY org.eclipse.jgit.pgm 4 0 2017-05-12 14:03:44.808
!MESSAGE FrameworkEvent ERROR
!STACK 0
org.osgi.framework.BundleException: Could not resolve module: org.eclipse.jgit.pgm [231]
  Unresolved requirement: Import-Package: org.eclipse.jgit.lfs.server.fs; version="[4.6.1,4.7.0)"
    -> Export-Package: org.eclipse.jgit.lfs.server.fs; bundle-symbolic-name="org.eclipse.jgit.lfs.server"; bundle-version="4.6.1.201703071140-r"; version="4.6.1"; uses:="javax.servlet,  javax.servlet.http,  org.eclipse.jgit.lfs.server,  org.eclipse.jgit.lfs.lib"
       org.eclipse.jgit.lfs.server [230]
         Unresolved requirement: Import-Package: com.google.gson; version="[2.2.4,3.0.0)"
  Unresolved requirement: Import-Package: org.eclipse.jgit.lfs.server; version="[4.6.1,4.7.0)"
    -> Export-Package: org.eclipse.jgit.lfs.server; bundle-symbolic-name="org.eclipse.jgit.lfs.server"; bundle-version="4.6.1.201703071140-r"; version="4.6.1"; uses:="javax.servlet.http,  org.eclipse.jgit.lfs.lib"
...
...
!ENTRY org.eclipse.wst.jsdt.js.node 4 0 2017-05-12 14:03:46.661
!MESSAGE FrameworkEvent ERROR
!STACK 0
org.osgi.framework.BundleException: Could not resolve module: org.eclipse.wst.jsdt.js.node [653]
  Unresolved requirement: Require-Bundle: com.google.gson; bundle-version="[2.2.4,3.0.0)"

Steps to reproduce:

1. Disable RHSCL 2.4 and Devstudio snapshot repo
2. Add RHSCL 2.3 and Devstudio stable repo
3. yum remove rh-*
4. yum install rh-eclipse46-devstudio (should install 10.2.0.GA)
5. check if eclipse works (I had no errors in the log)
6. Disable RHSCL 2.3 and Devstudio stable repo
7. Enable RHSCL 2.4 and Devstudio snapshot repo
8. rm -rf ~/.eclipse
9. Start the Eclipse with -clean
10. There should be error I mentioned above. I didn't see any impact to users, egit worked fine.
11. sudo yum update "rh-*"

    Updating:
     rh-java-common-google-gson                                  noarch                    2.2.4-1.el7                          rh-eclipse46-build                    177 k
     rh-java-common-javapackages-tools                           noarch                    4.3.2-1.14.el7                       rh-eclipse46-build                     95 k
     rh-java-common-lucene5                                      noarch                    5.4.1-2.3.el7                        rh-eclipse46-build                    2.2 M
     rh-java-common-lucene5-analysis                             noarch                    5.4.1-2.3.el7                        rh-eclipse46-build                    1.3 M
     rh-java-common-lucene5-analyzers-smartcn                    noarch                    5.4.1-2.3.el7                        rh-eclipse46-build                    3.4 M
     rh-java-common-lucene5-queries                              noarch                    5.4.1-2.3.el7                        rh-eclipse46-build                    211 k
     rh-java-common-lucene5-queryparser                          noarch                    5.4.1-2.3.el7                        rh-eclipse46-build                    339 k
     rh-java-common-lucene5-sandbox                              noarch                    5.4.1-2.3.el7                        rh-eclipse46-build                    246 k
     rh-java-common-python-javapackages                          noarch                    4.3.2-1.14.el7                       rh-eclipse46-build                     87 k
     rh-maven33-runtime      
    

12. Restart Eclipse, it will start without errors.