Fixed bug when disassociating route table from subnet.

The previous code would find the first route table association
and use it's ID to remove the association. This is incorrect
because AWS returns all associates in the DescribeRouteTables
API call.

New code checks the subnetId on the association first.
Changed ELBMethod->EC2Method on constant usage for correctness.
Merge pull request #127 from TheWeatherCompany/route-table-fix

Fixes route table disassociation bug.