Use dedicated api for lastCompilationTime in build definition

Since sbt 0.13.0, we have dedicated API in Analysis that gives access to
information about all compilations performed since the last time clean
was ran.

By looking at at last compilation we can determine lastCompilationTime
value.

See also sbt/sbt#1272
Merge pull request #477 from gkossakowski/lastCompilationTime

Use dedicated api for lastCompilationTime in build definition