Try travis_wait to extend timeout

Summary:
Builds have been timing out with Compiler: : #GHC 8.0.2 C. Try `travis_wait` which extends the timeout to 20 mins instead of 10.

The error has been -
```
Linking dist/build/duckling-test/duckling-test ...
The command "cabal build -j1" exited with 0.
$ cabal test -j1 --show-details=always
Preprocessing library duckling-0.1.6.1...
Preprocessing test suite 'duckling-test' for duckling-0.1.6.1...
Running 1 test suites...
Test suite duckling-test: RUNNING...
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
```

Reviewed By: blandinw

Differential Revision: D9020909

fbshipit-source-id: 7ccf8d3265860c3336b5191a5e89a493645bba2a

.travis.yml

@@ -73,7 +73,7 @@ script:
  - if [ -f configure.ac ]; then autoreconf -i; fi
  - cabal configure --enable-tests -v2  # -v2 provides useful information for debugging
  - cabal build -j1 # this builds all libraries and executables (including tests/benchmarks)
- - cabal test -j1 --show-details=always
+ - travis_wait cabal test -j1 --show-details=always
  - cabal check
  - cabal sdist   # tests that a source-distribution can be generated