Ensure byte code enhancer updates incremental compiler analysis

In SBT 0.13.1, a feature was added to the incremental compiler that
meant that products (class files) were invalidated if their modification
date changed, where previously the only check that was done was whether
they existed.

Play's byte code enhancement enhances Scala template classes, and this
enhancement was updating the modification date, naturally causing the
incremental compiler to invalidate the classes on its next run, and
recompiling them unnecessarily.

This change updates the analysis object after Play has modified any
template files, and then writes the updated analysis out to disk.
Merge pull request #2237 from jroper/enhancer-analysis-update

Ensure byte code enhancer updates incremental compiler analysis