Attempt both the PR author's branch and the PR's target branch. This resolves
issues on experimental where we need matrix-react-sdk to also be experimental.
A step towards a real solution for https://github.com/vector-im/riot-web/issues/7305
This approach makes use of `npm link` to remove the use of symlinks in the build process. The build process has also been altered to invoke the build process of each underlying SDK (react, js). This means that one can now `npm link` and `npm start` and have a working environment.
At the same time, parallelshell was dropped due to lack of maintenance from the maintainer.
* tell git to checkout the right branch, rather than cloning and then changing.
* clone with depth 1 under travis, to save time.
* less pushd/popd, which print out confusing text - use `cd` in a subshell
instead. (and just avoid it where possible).
* add some markers to the output to let travis roll it up.
