Specifiy exact include paths for WPEngine. Resolves #1.

The key here was in the difference between the path in the error message, `/usr/share/pear/Dropbox/autoload.php`, and the path requested in PHP, `"Dropbox/autoload.php"`.

When given a relative file location, PHP checks through the list of include paths for the file. In this case, it looks like a few things were happening:

1. `/usr/share/pear/` was listed in the include paths before `./` (current directory)
1. `/usr/share/pear/` included the Dropbox library.
1. WPEngine permissions allowed the file to be found, but not executed.

**Suggestions for WPEngine**
Fix permissions on the pear libraries!

**Fix in the plugin**
Specified the full path.

**Pedantic changes**
* Changed double quotes to single quotes. Double quotes should only be used in PHP for strings containing variables.
* Changed `require( 'file' )` syntax to `require 'file'`. PHP will let you do it either way, but `require` like `echo` is a language construct, not a function. PHP auto-converts `require()` to `require` for convenience (and to solve confusion), but the command is like `echo` or `return`, not `print_r()` or `trim()`. See http://php.net/manual/en/reserved.keywords.php