Relative instead of absolute paths

Previously, the buildpack used absolute paths everywhere, including in
the command that was run for the droplet.  This was incorrect as the
absolute path during staging is different than the absolute path at
runtime.  This changes the command to always use relative paths while
preserving absolute paths for the mutation work done as part of
compile.

[#49694799][#1]
Merge 49694799-hard-coded-paths to master

[Completes #49694799]
Gem Updates

There were some out of date Gems referenced.  This change updates
them.

[#51905291]
Merge 51905291-gem-updates to master

[Completes #51905291]
Builder documentation link

Previously there was no link from the java-buildpack documentation to
the OpenJDK builder documentation.  This made discovery difficult.
This change adds that link.

[#51721311]
Merge 51721311-openkjdk-builder to master

[Completes #51721311]
Builder documentation link

Previously there was no link from the java-buildpack documentation to
the Tomcat builder documentation.  This made discovery difficult. This
change adds that link.

[#51721311]
Merge 51721341-tomcat-builder to master

[Completes #51721341]
Test application documentation link

Previously there was no link from the java-buildpack documentation to
the Java Test Application documentation.  This made discovery
difficult. This change adds that link.

[#51721311]
Merge 51720893-test-applications to master

[Completes #51720893]
Merge remote-tracking branch 'upstream/master'