Use environment variable instead of string for API key

After working with the tests for awhile, I got a little bored with
remembering to remove my API key. Using an environment variable
makes it easier to make updates to the tests and makes sure no
one accidentally commits their key.

I also considered using a config file but there was too much
overhead.
Updated to reflect testing changes
Updated League to version 2.5

The new version removes several fields from the `LeagueItem` struct
that already appear in `League`. It also has new capabilities including

* support for Teams
* selecting up to 10 Summoners or Teams

With the addition of looking at Leagues by Team, I've added a function
that concatenates a slice of TeamIDs to a comma seperated string. It
works identically to `createSummonerIDString`.

Tests for the new Team interface in `league.go` have been added.
Merge branch 'env-var' into matt-master
Updated Team to 2.4

The latest version of the Team API allows a client
to request up to 10 summoners at a time. There is also the ability
to search for a team based on their TeamID or up to 10 Teams.

I've also added a function to create comma seperated strings
based on TeamIDs. This is very similar to `createSummonerIDString`
except that it takes a `[]string` as an argument versus a `[]int64`.

A test has been added for `TeamByTeamID`.
Merge branch 'matt-master' into team-new-master