Babel 7 compatibility (Fixes #193) (#194)

* Add dependencies on babel-types and babel-helper-module-imports

* Traverse file AST ourselves to build module metadata.

Babel no longer does this internally as of babel/babel#6343. This commit implements the same
traversal as part of this plugin, using the same code that was removed from Babel with some
trivial modifications.

* Use addDefault from babel-helper-module-imports to add our imports.

file.addImport is deprecated by Babel and attempting to use it will throw.

Author: jscinoz