Always spell adapter with a e

[brucek]

Fixes #275

This pull introduces/changes:

• changes spelling of adaptor to adapter (with an 'e') everywhere to match Faraday gem and standard US English

Pings:
@cheerfulstoic
@subvertallchris

[brucek]

Again, you may not want to do this, just putting it out there since it was a bit of a pain when switching back and forth with Faraday.

[coveralls]

Coverage remained the same at 92.361% when pulling d8be441 on brucek:adapter_with_an_e into 7abe23c on neo4jrb:master.