fix panic: failed to determine gopath: exec: "go" (Refs #782)

```
-bash-4.2$ tendermint show_validators
panic: failed to determine gopath: exec: "go": executable file not found in $PATH

goroutine 1 [running]:
github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.gopath(0xc4200632c0, 0x18)
	/var/lib/jenkins/workspace/03.Build.Package/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/os.go:26 +0x1b5
github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.init()
	/var/lib/jenkins/workspace/03.Build.Package/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/os.go:17 +0x13c
github.com/tendermint/tendermint/vendor/github.com/tendermint/go-wire.init()
	/var/lib/jenkins/workspace/03.Build.Package/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-wire/wire.go:165 +0x50
github.com/tendermint/tendermint/vendor/github.com/tendermint/go-wire/data.init()
	/var/lib/jenkins/workspace/03.Build.Package/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-wire/data/wrapper.go:89 +0x50
github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/cli.init()
	/var/lib/jenkins/workspace/03.Build.Package/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/cli/setup.go:190 +0x76
main.init()
	/var/lib/jenkins/workspace/03.Build.Package/go/src/github.com/tendermint/tendermint/cmd/tendermint/main.go:42 +0x49```

An error message instead would be nice.
```

Now GoPath() is a function instead of a variable.

consensus/replay_test.go

@@ -44,7 +44,7 @@ func init() {
 // after running it (eg. sometimes small_block2 will have 5 block parts, sometimes 6).
 // It should only have to be re-run if there is some breaking change to the consensus data structures (eg. blocks, votes)
 // or to the behaviour of the app (eg. computes app hash differently)
-var data_dir = path.Join(cmn.GoPath, "src/github.com/tendermint/tendermint/consensus", "test_data")
+var data_dir = path.Join(cmn.GoPath(), "src/github.com/tendermint/tendermint/consensus", "test_data")
 
 //------------------------------------------------------------------------------------------
 // WAL Tests

glide.lock

@@ -1,5 +1,5 @@
 hash: 9867fa4543ca4daea1a96a3883a7f483819c067ca34ed6d3aa67aace4a289e93
-updated: 2017-10-23T10:01:08.326324082-04:00
+updated: 2017-10-25T07:15:06.075544403Z
 imports:
 - name: github.com/btcsuite/btcd
   version: c7588cbf7690cd9f047a28efa2dcd8f2435a4e5e
@@ -122,7 +122,7 @@ imports:
   subpackages:
   - iavl
 - name: github.com/tendermint/tmlibs
-  version: 8e5266a9ef2527e68a1571f932db8228a331b556
+  version: 0a652499ead7cd20a57a6a592f0491a2b493bb85
   subpackages:
   - autofile
   - cli