Add specs for ENV#set including failing specifications for Ruby "basic types" implicitly converted by YAML

This commit addresses issue #13.

Since Ruby's implementation of YAML has a set of ["basic types"](http://www.yaml.org/YAML_for_ruby.html#basic_types) that are implicitly converted from String to the appropriate type, Figs should convert the "basic types" to strings and store them in ENV, so that a calling application can just call ENV['VAR_NAME'] for "basic types", which is more intuitive.

Basic types handled by YAML and included in the specifications for ENV#set:
- Integer
- Float
- Null
- Boolean
- Time (in the ISO8601 format)
- Date (in the ISO8601 format)
Add a case statement in ENV#set for handling Ruby YAML "basic types"

The case statement converts values passed to ENV#set for "basic type" classes to their appropriate String representations. This commit update adds the ability to call ENV['VAR_NAME'] from a Figs application for "basic types" and fixes issue #13.

The "basic types" handled in ENV#set are
- Integer
- Float
- Null
- Boolean
- Time
- Date
Bugfix by scotdalton
Merge pull request #14 from NYULibraries/feature/ruby-yaml-basic-type-conversion

Ruby YAML "basic type" conversion