Go version 1.2 is now a minimum requirement

Since the TLS TcpOutput was added in #634 version 1.2 is a minimum
requirement. Travis CI is already setup to use 1.2 so change is needed
there.

The error that appears:

```
# github.com/mozilla-services/heka/plugins/tcp
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:28: undefined: tls.TLS_ECDHE_ECDSA_WITH_RC4_128_SHA
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:29: undefined: tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:30: undefined: tls.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:35: undefined: tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:36: undefined: tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:40: undefined: tls.VersionSSL30
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:41: undefined: tls.VersionTLS10
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:42: undefined: tls.VersionTLS11
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:43: undefined: tls.VersionTLS12
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:86: goConf.MinVersion undefined (type *tls.Config has no field or method MinVersion)
heka/src/github.com/mozilla-services/heka/plugins/tcp/tls.go:86: too many errors
make[2]: *** [CMakeFiles/hekad] Error 2
make[1]: *** [CMakeFiles/hekad.dir/all] Error 2
make: *** [all] Error 2
```