(maint) Update very old Beaker version in Gemfile

[glennsarti]

Currently beaker is pinned at version 2.x which causes issues on Windows with
modern Ruby. Beaker 2.x pulls in VERY old versions of nokogiri which cannot
be compiled on Windows. This commit updates the Beaker version pin to version
3.x as this has been out for over a year, and will allow developers on Windows
to at least do a bundle install. Later commits may update this to the latest
beaker version (4.x)

[glennsarti]

Note the very old nokogiri version

Installing json 1.8.6 with native extensions
Fetching nokogiri 1.5.11
Installing nokogiri 1.5.11 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
C:/Source/puppet-strings/.bundle/windows/ruby/2.5.0/gems/nokogiri-1.5.11/ext/nokogiri
C:/tools/ruby2.5.1-2x64/bin/ruby.exe -r ./siteconf20190618-29788-cv9l2d.rb
extconf.rb
checking for libxml/parser.h... no
-----
libxml2 is missing.  please visit
http://nokogiri.org/tutorials/installing_nokogiri.html for help with installing
dependencies.
-----
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

...


An error occurred while installing nokogiri (1.5.11), and Bundler cannot
continue.
Make sure that `gem install nokogiri -v '1.5.11' --source
'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  beaker-rspec was resolved to 2.0.0, which depends on
    beaker was resolved to 1.16.0, which depends on
      aws-sdk was resolved to 1.42.0, which depends on
        nokogiri

[scotje]

Rebased and updated beaker pin to ~> 3.37

[glennsarti]

@scotje The ruby version was specifically set at 2.1.0 due to hhttps://github.com/puppetlabs/puppet-strings/commit/2dfc8073b36f38690475a9cc6e004bf065e90cdc