The embed-core JAR from Tomcat 8.0 and 7.0 don't include the JULI classes, which is a runtime requirement. Tomcat 8.5 and 9 now include these classes in embed-core.
To be compatible with upcoming 8.5 and 9 in the scope of Spring Boot 1.4, and since the embed JARs were never published before, I recommend including the juli classes in embed-core.

The patch to do that would be (for Tomcat 8, and for 7 it's almost identical):

Index: build.xml
===================================================================
— build.xml (revision 1767623)
+++ build.xml (working copy)
@@ -443,6 +443,7 @@
<patternset refid="files.catalina" />
<patternset refid="files.servlet-api" />
<patternset refid="files.tomcat-api" />
+ <patternset refid="files.tomcat-juli" />
<!-- These pattern sets conflict so include files directly
<patternset refid="files.tomcat-coyote" />
<patternset refid="files.tomcat-util" />