Specified UTF-8 collation for sessions table

Fixes this error: "Error: ER_TOO_LONG_KEY: Specified key was too long; max key length is 767 bytes"

255 character ids do not fit within InnoDB's 767 bytes key limit if the database has a default collation of utf8mb4_general_ci (common for databases that support Emoji).
Merge pull request #13 from jeremiahlee/max-key-length-fix

Specified UTF-8 collation for sessions table