Move @types to devDependencies

[andy-hanson]

diff and semver are only used for testing, although test.ts is in src. Should they be devDependencies too?

[ajafff]

diff and semver can not be moved to devDependencies.
Once you installed tslint, you can always run tslint --test which uses both libraries. That's actually how I test my custom rules. It would be pretty inconvenient to have to install both packages myself.

[nchen63]

@andy-hanson thanks!