Ensure build.sbt ends in newline

Rationale: Unix tools (including git) hate files which don't end with a newline.

To allow this, remove the call to trim(). I verified that the resulting
build.sbt contains no superfluous whitespace. In fact, before this
commit trim had no effect since its argument was already trimmed.
Merge pull request #8 from Blaisorblade/topic/newline

Ensure build.sbt ends in newline