Don't record deps in dry runs.

deps_log() is NULL during dry runs, so this fixes a crash. It also
matches ninja 1.2.0's behavior as far as I can tell.

Fixes issue #551.
keep dry run more similar to normal run
Merge pull request #552 from nico/deps_dryrun

Don't record deps in dry runs.
add a .travis.yml, trying out travis-ci.org
travis: pull in gtest before building
make DepsLogTest actually clean up on teardown

I think I commented this out when tracking down a bug and forgot
to uncomment it.
add a test for truncated deps

This doesn't yet exhibit the full problems with truncation, but it's
a start.
disable SubprocessTest.SetWithLots on travis-ci

We run into fork() limits on their VM.
deps log: recover on truncated entry

If a read fails while reading an entry, truncate the log to the last
successfully read entry.  This prevents corruption when a subsequent
run appends another entry.
fix warning
don't count eof as truncated