Fix Travis CI config

[hugovk]

Something changed at Travis CI which meant the matrix expanded to an empty list due to the combination of:

# Default Python version is usually 2.7
python: 3.5

And:

jobs:
  exclude:
      # Exclude the default Python 3.5 build
      - python: 3.5

See: #141 (comment)

Removing the exclusion fixes it.

Last run	This one

[radarhere]

So, #133 is happening again

The workaround for that issue was to switch to Xcode 9.3. For some reason, Travis is here running 9.4 which is not what we're specifying.