travis: Don't disable writing ccache for pull-requests

This was doing more harm than good. The original intention was to speed up
builds, since a PR's ccache results will be thrown away anyway.

However, each PR maintains its own cache, so disabling writes means that
subsequent pushes don't benefit from the fresh cache. This is significant when
(for example) many headers are touched in a PR, then the PR is updated. With
this change, the updated PR will take advantage of the cache generated during
the PR's previous build.