hashie/mash can be required alone, require it's dependencies

While it's probably not something hashie wants to allow, previously
this was possible so we shouldn't break this when going 3.4.4 to 3.4.5.

As an example, omniauth requires hashie/mash alone:
https://github.com/omniauth/omniauth/blob/d941c4bcb1eb52f3674dd46225808751dd6a1c2f/lib/omniauth/auth_hash.rb#L1

This appears to have been broken in #358.

Note, we can't write a test for this because we require 'hashie' in the spec_helper
which properly defines the Hashie::Extensions::Array constant and autoloads
the pretty_inspect here:
https://github.com/intridea/hashie/blob/229ee36d7c6a07eff6d8a0434726e12b8c3a0223/lib/hashie.rb#L48

**Before**

```
10:44:30 ~/Code/hashie (master) (2.3.1) + be irb
irb(main):001:0> require 'hashie/mash'
NameError: uninitialized constant Hashie::Extensions::Array
Did you mean?  Hashie::Array
               Array
  from /Users/joerafaniello/Code/hashie/lib/hashie/array.rb:3:in `<class:Array>'
  from /Users/joerafaniello/Code/hashie/lib/hashie/array.rb:2:in `<module:Hashie>'
  from /Users/joerafaniello/Code/hashie/lib/hashie/array.rb:1:in `<top (required)>'
  from /Users/joerafaniello/Code/hashie/lib/hashie/mash.rb:2:in `require'
  from /Users/joerafaniello/Code/hashie/lib/hashie/mash.rb:2:in `<top (required)>'
  ...
```

**After**

```
10:44:40 ~/Code/hashie (master) (2.3.1) + be irb
irb(main):001:0> require 'hashie/mash'
=> true
irb(main):002:0> exit
```

lib/hashie/array.rb

@@ -1,3 +1,6 @@
+require 'hashie/extensions/array/pretty_inspect'
+require 'hashie/extensions/ruby_version_check'
+
 module Hashie
   class Array < ::Array
     include Hashie::Extensions::Array::PrettyInspect