Friendlier message when using a type checker that is not a function

[gouegd]

Just a text change, adds a stronger hint as to the most common issue.
This tries to prevent confusion as discussed in #33

Sample:

> PropTypes.checkPropTypes({pname: PropTypes.stringzz}, {pname: false}, 'prop', 'MyCompo')
Warning: Failed prop type: MyCompo: type specification of prop `pname` is invalid; the type checker must be a function but is `undefined`. This often happens because of typos such as `React.PropTypes.function` instead of `React.PropTypes.func`.

Fixes #33.

[kevincoleman]

Bump.

Ran into this little confusion; googled it, and found this whole issue/PR thread. Simple change, just merge!

[ljharb]

(07d1b47 added the "received" type)