Only build on PRs

Rebuilding on master takes up extra CI time, since we already successfully built things for them to get merged.

To avoid this we could:
1. build the package in the PR as usuall
1. if the build succeeds push the package to a staging area for the package
1. when the PR is merged move the package from the staging into the main

The main issues that need to be tackled to implement this are:
- [ ] Finding the correct package to move, we may produce a lot of packages with very similar names so picking among them might be an issue (maybe pick the latest)
- [ ] Make a staging area, using a staging account so that attackers can't reveal our tokens.
- [ ] Make something to do the moving, maybe a single Circle job or a webservice listening for merged PRs.
- [ ] Require CI to pass for merging, since we don't build on master we can't merge failing PRs.

ping @scopatz