Error on Mac configuring PyPI via Travis

## Location within the Course

* Section:4
* Lesson:4
* Language:English

## Description

In "Configure PyPI Authentication Via Travis". I'm on a Mac and installed Travis using the command `brew install travis`, but then when I did `travis encrypt --add deploy.password`, I get this message:
```
Shell completion not installed. Would you like to install it now? |y y
Detected repository as purplediane/shortbread, is this correct? |yes| yes
not logged in, please run travis login --pro
```
Then I did `travis login --pro`
It asked for username and password, which I entered, and then got:
```
no implicit conversion of nil into String
for a full error report, run travis report --pro
```
Turns out the problem has something to do with ruby versions. When I do `ruby --version`, it tells me I have 2.5.0p0, but my travis report said it is 2.0.0-p648, which is not current enough to work. Probably my Homebrew is not quite right, since I rarely use it, although I recently did `brew update` or whatever you are supposed to do to keep the Brewed things up-to-date. (I try not to be too much of a curmudgeonly luddite, but just can't get interested in Homebrew enough to spend the time to understand it; sorry, brew-lovers).

### Solution
My solution was to `brew uninstall travis` and then do `gem install travis -v 1.8.8 --no-rdoc --no-ri` per [instructions here](https://github.com/travis-ci/travis.rb#installation). Even though it claims that Ruby 2.0 is OK, others have said the problem went away after upgrading Ruby.

In any case, after installing it as a gem, the `travis encrypt` command worked.

See https://github.com/travis-ci/travis.rb/issues/190#issuecomment-369749402 for more discussion of this issue.

Maybe this can be in the FAQ or troubleshooting area, with a link from the location in 4.4 for those who have the problem.

Start with Section 4, Lesson 4, especially “Configure PyPI Authentication Via Travis,” and review the linked travis.rb issue for the reported Mac setup failure. Add a troubleshooting or FAQ note covering the working installation approach and link it from the lesson; done means affected readers can find the workaround.

Written by the indexing model from the issue text.