Set sourcelevel to 1.7.
Set sourcelevel to 1.7.

This is similar to what we recently did for Guava: https://github.com/google/guava/commit/118ac49cddb47362b28cfd893f67791809d60679

It's a way to get GWT working again with Java 1.8 without updating GWT to 2.7.0. The downsides of updating GWT are:
- (completely speculative) Users might not have updated yet, and perhaps that's a problem somehow?
- (not directly related to GWT but part of the same commit that I reverted) I'm seeing problems resolving AutoValue 1.0-SNAPSHOT. (I imagine that we could fix this by pushing a new version. Do we intend to have that set up to happen automatically with each successful build?)
- For dependency reasons, we had to back out our workaround for slow group resolution. This is probably OK when we building with JDK8, but the problem still exists for anyone building with JDK7.
- GWT now gets the wrong version of ASM, which makes our GWT tests fail. (We might be able to solve the dependency knot. I didn't investigate.) https://plus.google.com/106802216066771790262/posts/RbVEDY8QfKc?e=-RedirectToSandbox

hcoles/truth,kidaa/truth,dahlstrom-g/truth,google/truth,cgruber/truth,google/truth,hcoles/truth,cgruber/truth,kidaa/truth,dahlstrom-g/truth,google/truth