check-exercises, ensure-stubs-compile: Make them run in the travis weekly (they currently don't)

[petertseng]

We build the master branch in Travis to make sure everything is good.

We'd like that to test every exercise. This was the approach agreed upon in #591 (comment) .

I present the most recent build of master, https://travis-ci.org/exercism/rust/jobs/455623555, where it says "No exercise was modified. The script is aborted."

Further, I present a build where I forced every exercise to be checked https://travis-ci.org/exercism/rust/jobs/456251504 for PR #727

Note the errors

hexadecimal's stub does not compile; please make it compile or remove all non-commented lines
nucleotide-codons's stub does not compile; please make it compile or remove all non-commented lines
Exercises that don't compile:
hexadecimal
nucleotide-codons

These two pieces together tell us that Travis CI's master build does not check exercises. As noted above, it should.

---

the below information is irrelevant for this issue:

We build the master branch weekly instead of on every push to master, because exercism/discussions#174 (comment) turned them off.
However, we do have the power to turn them back on if we so choose.
However, I think this detail is irrelevant to the issue at hand. No matter the mechanism of building master, it is currently the case that building master means no exercise is checked.

[petertseng]

Try using some of the https://docs.travis-ci.com/user/environment-variables/#default-environment-variables to do this.