Don't cache target/release folder on travis

[brson]

I suspect this is causing the following error I'm seeing during deploy:

sh: 1: cannot create target/i686-unknown-linux-gnu/release/cli_inst_interactive-f17347ea614c070e.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256.sha256: File name too long

I don't know why this folder needs to be cached anyway.

r? @alexcrichton

[alexcrichton]

@bors: r+

[bors]

📌 Commit 990ca18 has been approved by alexcrichton

[bors]

⌛ Testing commit 990ca18 with merge 2d8e6c0...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 2d8e6c0 to master...