Remove AMP from the front of the form action

Apparently, EE2.8 can't handle that leading AMP, but if you take it off,
EE2.8 will rewrite the URL to be the new EE2.8 style.

Removing it does not affect EE2.7.
Use EE2.8's new Cache class, when available

ExpressionEngine added a new Cache[1] class in EE2.8 that is specifically
for persistent caches. ExpressionEngine would like modules to move toward
this, presumably so it can give users more control of cached items.

For this module, that means putting the result of the Twitter API call in
the cache instead of writing to a file.

For EE older than 2.8.0, we still cache to a file.

[1]: http://ellislab.com/expressionengine/user-guide/development/usage/cache.html
Change version to '1.7.0'