Update README.md

Add explanation for version parity.
Update gem version to v0.7.4.
Readme: fix duplication of #map elment in DOM

`map()` automatically creates a #map div, there is no need to wrap it in another one.
Merge pull request #34 from nedeco/readme-patch1

Readme: fix duplication of #map elment in DOM
Merge branch 'master' of github.com:axyjo/leaflet-rails