Make assertReleaseNeeded task more informative

[epeee]

fixes #209

[mockitoguy]

Let's tweak the output a bit before merging. Here's the current output as tested with your branch:

:releaseNeeded
  Environment variable SKIP_RELEASE present: false
  Commit message to inspect for keywords '[ci skip-release]' and '[ci skip-compare-publications]': <unknown commit message>
  Current branch 'ep' matches 'master|release/.+': false

  Compared 1 publication(s). Changes since previous release:
  Differences between files:
  --- /Users/sfaber/mockito/release/subprojects/shipkit/build/previous-release-artifacts/shipkit-0.9.145-sources.jar
  +++ /Users/sfaber/mockito/release/subprojects/shipkit/build/libs/shipkit-0.9.146-sources.jar

    Modified files:
    +- org/shipkit/internal/gradle/release/tasks/ReleaseNeeded.java


Release is considered _not_ needed when:
 - 'SKIP_RELEASE' environment variable is present (currently: false)
 - commit message contains '[ci skip-release]' (currently: false)
 - we are building a "pull request" (currently: false)
Release is needed when all above is false and: 
 - we are building on a branch that matches regex 'master|release/.+' (currently: false)
: - and one of the following criteria is true:
   - there are changes in the binaries when compared to previous version (currently: true)
   - commit message contains '[ci skip-compare-publications]' (currently: false)
   - 'skipComparePublications' property on task releaseNeeded is true (currently: false)

 Skipping release because we are not on a releasable branch.

Feedback:

• I love the improved explanation!!!
• "Environment variable SKIP_RELEASE present: false" text at the beginning is redundant, let's remove it and just keep the new explanation you implemented.
• there is an awkward ": -" in the middle of the bullet list

I'm approving because above are small cosmetic fixes, the change is really nice! It will improve usability of Shipkit. I found myself many times figuring out why the release was skipped :)

THANKS!