Configure Travis builds for branches and PRs

[tombruijn]

Only build master and develop branches when pushed. When a PR is made on GitHub travis will start a build as well.

This change prevents two builds being started for one change, like you can see in PR #61 under "All checks have passed - 2 successful checks".

This PR only triggers one build as you can see below.

Scenario

Steps to reproduce issue

• I push a branch to make a PR (a build is triggered on TravisCI)
• I make a PR on the repo (a build is triggered on TravisCI)

expected result

• one build triggered

actual result

• two builds triggered

New setup

• build branches "master" and "develop" on push
• build PRs when created.
• other branches are ignored on push

Note

This is the same behavior we have on the Ruby gem repo: appsignal/appsignal-ruby#131