Fix conversion error for EMR waiters

Found by @someboredkiddo
Introduced in 23e042ff8716003ee53d62698668a95fe318c4b8

Closes #378
Fixes aws/aws-cli#1007

You can see the diff here: https://github.com/boto/botocore/commit/23e042ff8716003ee53d62698668a95fe318c4b8#diff-561cbc8ca2199768839320b0a5e2f0f0L14

cc @kyleknap @danielgtaylor