travis: Build with --opt-level=1 instead of 2

The most frequent failure for our travis builds is running into the timeout
limits when building the compiler itself. Building librustc takes a very long
amount of time, often hitting the 10 minutes with no output threshold that
travis imposes on us.

This commit switches to bootstrapping with --opt-level=1 which should maintain a
reasonable level of performance of the stage1 compiler while being able to build
librustc in a more reasonable amount of time.

I ran this through travis, and gathered these statistics:

* The overall build time increased by 4 minutes from 29m to 33m
* The build time of rustc dropped from 9.5m to 6m

This will hopefully enable us to have fewer flaky builds on travis, while still
remaining in the window that travis gives us for the overall build time.
Additionally, this is one of the only places that we'll be testing --opt-level=1
compilers, so perhaps it's not a bad testing ground!