Remove support for macOS X Sierra (10.12) in Travis CI build

[gedakc]

The Travis CI builds for macOS X are failing because there are no
Homebrew bottles (packages) for macOS X Sierra (10.12).

The error message in the Travis CI log is:

...
# Upgrade to python 3.x
brew upgrade python
Warning: You are using macOS 10.12.
We (and Apple) do not provide support for this old version.
...

Note that by default the Travis CI will terminate a build after 10
minutes if no output has been received. Unfortunately building qt
from source code may take hours, so installation using a Homebrew
bottle is required.

To address this issue, choose a macOS X version that has a Homebrew
bottle for qt [1].

[1] https://formulae.brew.sh/formula/qt

The homebrew project currently lists having a qt bottle for:

• Catalina (10.15)
• Mojave (10.14)
• High Sierra (10.13)

No bottle is listed for macOS Sierra (10.12)

In order to support as many versions of macOS X as possible, choose
the lowest osx_image value [2] that is currently supported with a
homebrew bottle.

[2] https://docs.travis-ci.com/user/reference/osx#os-x-version

At this time osx_image value xcode9.3 is listed as supporting OS X
10.13.

Unfortunately xcode9.3 did not work in the Travis CI build.

By following the suggestions for troubleshooting homebrew [3] a
discovery was made that a higher value of xcode10.1 was required.

[3] https://docs.brew.sh/Troubleshooting

Suggestion was to use "brew update" twice and "brew doctor" twice.

The messages in the log were as follows:

...
Warning: Your Xcode (9.3) is outdated.
Please update to Xcode 10.1 (or delete it).
...

Now try setting osx_image value to xcode10.1 which is listed as
supporting OS X 10.13.

NOTE: While this PR does not succeed in creating a new macOS X PyInstaller package, it does address the issue of Apple and Homebrew dropping support for macOS X Sierra (10.12). As such I plan to merge this PR.

The problem with the failure to create a macOS X PyInstaller package requires a macOS X person to investigate and fix.

[gedakc]

Merging this PR.

Note that help is needed with maintaining the macOS X package and the instructions to Run Manuskript from Source Code on Mac OS X.