check-disk-usage.rb fails on FreeBSD

[UnitedMarsupials-zz]

As transpired in the course of discussing #56, the recent versions of the check-disk-usage.rb do not work on FreeBSD:

% check-disk-usage.rb 
Check failed to run: undefined method `[]' for nil:NilClass, ["/opt/lib/ruby/gems/2.2/gems/sensu-plugins-disk-checks-2.1.0/bin/check-disk-usage.rb:185:in `to_human'", "/opt/lib/ruby/gems/2.2/gems/sensu-plugins-disk-checks-2.1.0/bin/check-disk-usage.rb:173:in `check_mount'", "/opt/lib/ruby/gems/2.2/gems/sensu-plugins-disk-checks-2.1.0/bin/check-disk-usage.rb:132:in `block in fs_mounts'", "/opt/lib/ruby/gems/2.2/gems/sensu-plugins-disk-checks-2.1.0/bin/check-disk-usage.rb:121:in `each'", "/opt/lib/ruby/gems/2.2/gems/sensu-plugins-disk-checks-2.1.0/bin/check-disk-usage.rb:121:in `fs_mounts'", "/opt/lib/ruby/gems/2.2/gems/sensu-plugins-disk-checks-2.1.0/bin/check-disk-usage.rb:209:in `run'", "/opt/lib/ruby/gems/2.2/gems/sensu-plugin-1.2.0/lib/sensu-plugin/cli.rb:56:in `block in <class:CLI>'"]

(/opt is the prefix on my systems -- the usual value is /usr/local)

The last version that worked was 1.1.2. Creating this new ticket on request of @majormoses. If necessary, I can create an account on my FreeBSD system for a developer -- contact me directly with the desiredpasswdentry and ssh public key.