chore(travis): revert blacklist Dart SDK 1.7.0-dev.2.0

This reverts commit e2350cac61dc33053504c70be4d1d5411e0f2040.

Current Dart dev sdk version has advanced past the buggy version.

Closes #1447
chore(travis): remove dart version blacklist for 1.7.0-dev.1.0

Closes #1443