Don't do things that set -e in Travis's shell

Sourcing files that set flags bleed into Travis' wrapper script, which
messes with their workflow, e.g. preventing after_failure scripts from
triggering.

These changes fix that:

* Move exported variables from install.sh to setup.sh, so install.sh
  doesn't have to be sourced.
* Source setup.sh from run.sh, rather than directly from .travis.yml.

In addition, as a good practice, remove `set -e` etc. from setup.sh,
and add a comment not to add them back. It's the sourcing script's
responsibility to set these flags.

scripts/linux/install.sh

@@ -5,7 +5,7 @@ set -o pipefail
 
 mapbox_time "checkout_mason" \
 git submodule update --init .mason
-export PATH="`pwd`/.mason:${PATH}" MASON_DIR="`pwd`/.mason"
 
+PATH="`pwd`/.mason:${PATH}" MASON_DIR="`pwd`/.mason" \
 mapbox_time "install_mesa" \
 mason install mesa 10.4.3