Update Ruby version, remove Travis workaround, and add Markdown linter

[harding]

This removes the need for the workaround in 71e9fad by updating Ruby. I chose Ruby 2.5.1 because that's the version used for Ubuntu 18.04 LTS, so users of that system should, in theory, not even need to use rvm to get a Ruby to build site previews. I use Debian, not Ubuntu, so I haven't tested that.

This also adds a lint checker suggested by @practicalswift with a single check for extraneous spaces at the end of a line. I think this is an important rule because some Markdown parsers turn two spaces at the end of a line into a <br> tag, complicating review because whitespace at the end of a line is often invisible. There are plenty of other rules we can add if we want, although I'd prefer to focus on preventing problems rather than enforcing a consistent style.

The short steps to upgrade are:

1. cd to your checkout
2. (Optional) Checkout current master, run make all, then run cp -a _site _old_site
3. Checkout this PR
4. Then,

rvm install 2.5.1
cd ; cd -
ruby --version
# Should say something like: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
bundle install
make all

Optionally, you can finish by running diff -ru _old_site _site to see how this change affects the build. Besides changing in-file datestamps, the only changes I see are rearragements in the order associative arrays are printed and the README.md file is no longer rendered on the site (as expected).

Note that if you checkout out a branch that uses the previous version of Ruby, you need to exit and re-enter the top-level repository directory to get RVM to select the current Ruby version. You can go to your home directory and back to your last-used directory using: cd ; cd -

On my system, going back to an older version of Ruby causes one of HTMLproofer's dependencies to segfault. This can be resolved by deleting your cached bundler gems (on Linux, try mv ~/.gems /tmp) and then re-running bundle install

[jnewbery]

Looks great. Tested ACK f3103ce.

As suggested here: #99 (comment), I'll merge this after next week's newsletter is merged.

[jnewbery]

#101 is merged, so merging this too.