fix(travis): prevent double builds on PRs

[tlvince]

Without this, if a PR is opened for a branch, Travis creates two builds; one for the branch and one for the PR itself. When Travis is configured (in the UI) to build both the push and PR (the default), I think a better default for builds is to trigger only one build.

Semantic release itself uses this set up, see: https://github.com/semantic-release/commit-analyzer/issues/11