Panics on term error if TERM=dumb

[sfackler]

$ env TERM=dumb RUST_BACKTRACE=1 cargo test
     Running target/debug/docker-be333b549b978cdb

running 1 test
thread '<main>' panicked at 'io error when running tests: Error { repr: Custom(Custom { kind: Other, error: ColorOutOfRange }) }', /Users/sfackler/.cargo/registry/src/git.luolix.top-88ac128001ac3a9a/rustc-test-0.1.3/src/lib.rs:254
stack backtrace:
   1:        0x102b81118 - sys::backtrace::tracing::imp::write::h3b28049ffa6d6406ZZu
   2:        0x102b844b5 - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.43942
   3:        0x102b840d8 - panicking::default_handler::h9792cd244a79d9daorz
   4:        0x102b6fca6 - sys_common::unwind::begin_unwind_inner::h127fbab41243a988LYt
   5:        0x102b7015e - sys_common::unwind::begin_unwind_fmt::h849e306453bce5feRXt
   6:        0x10294e51b - test_main::h097220cc7474210a35a
   7:        0x1027fe557 - main::hdafe840e8602d59bBaa
   8:        0x102b83ce2 - sys_common::unwind::try::try_fn::h6504634030632813459
   9:        0x102b805cb - __rust_try
  10:        0x102b83ba0 - rt::lang_start::h9c1978904cd16d9cujz
  11:        0x102806b49 - main