Update pip install instructions with quotes for better compatibility.

[carlosperate]

Another small PR, this time updating the pip instructions as concluded in #852.

[ntoll]

Thanks @carlosperate 👍

[carlosperate]

About the travis error, it happens when briefcase it's trying to package:

Packaging Mu into a macOS native application.
python setup.py macos --support-pkg=https://github.com/mu-editor/mu_portable_python_macos/releases/download/0.0.6/python3-reduced.tar.gz
/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/site-packages/setuptools/dist.py:472: UserWarning: Normalizing '1.1.0.alpha.1' to '1.1.0a1'
  normalized_version,
running macos
 * Writing application template...
Project template: https://github.com/pybee/Python-macOS-template.git
Traceback (most recent call last):
  File "setup.py", line 135, in <module>
    'icon': 'package/icons/mac_icon',
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/site-packages/setuptools/__init__.py", line 145, in setup
    return distutils.core.setup(**attrs)
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/distutils/core.py", line 148, in setup
    dist.run_commands()
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/distutils/dist.py", line 955, in run_commands
    self.run_command(cmd)
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/distutils/dist.py", line 974, in run_command
    cmd_obj.run()
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/site-packages/briefcase/app.py", line 514, in run
    self.generate_app_template()
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/site-packages/briefcase/app.py", line 264, in generate_app_template
    extra_context=_extra_context
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/site-packages/cookiecutter/main.py", line 63, in cookiecutter
    password=password
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/site-packages/cookiecutter/repository.py", line 103, in determine_repo_dir
    no_input=no_input,
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/site-packages/cookiecutter/vcs.py", line 99, in clone
    stderr=subprocess.STDOUT,
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/subprocess.py", line 336, in check_output
    **kwargs).stdout
  File "/Users/travis/.pyenv/versions/3.6.5/lib/python3.6/subprocess.py", line 418, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['git', 'clone', 'https://github.com/pybee/Python-macOS-template.git']' returned non-zero exit status 128.
make: *** [macos] Error 1
The command "if [ "$TRAVIS_OS_NAME" = "osx" ]; then make macos; fi" exited with 2.

Git cannot clone https://github.com/pybee/Python-macOS-template.git.
The org is now called beeware and the right URL should be https://github.com/beeware/Python-macOS-template.git, but I though GitHub automatically redirected these renames. Maybe we need to update to a newer version of briefcase?

[ntoll]

Yeah... @carlosperate I've seen this several times and it's Travis being non-deterministic. I generally restart and it goes green.