Update travis with python/cask deprecation.

Travis is failing:
https://travis-ci.org/github/jorgenschaefer/pyvenv/jobs/772569402, so trying to
work around the deprecation:

```
...
Done (Total of 2 files compiled, 1 skipped)
Package `s-1.12.0' deleted.
Wrote /home/travis/.emacs.d/.cask/24.3/bootstrap/s-20180406.808/s.el
File exists: /home/travis/.emacs.d/.cask/24.3/bootstrap/s-20180406.808/s-pkg.el

!!!
!!!                    DEPRECATION NOTICE
!!!
!!!
!!!    The cask `go` script will be removed on 2021/06/01.
!!!
!!!    This is due to security concerns about the way python is
!!!    invoked from curl, and to remove the python dependency from cask.
!!!
!!!    The way to install cask without depending on the `go` script
!!!    is very simple.  Just clone Cask and pass the PATH.
!!!
!!!        git clone https://github.com/cask/cask ~/.cask
!!!        PATH=$HOME/.cask/bin:$PATH
!!!
!!!        # If you want to make it permanent
!!!        echo 'PATH=$HOME/.cask/bin:$PATH' >> .bashrc
!!!

Cask could not be bootstrapped. Try again later, or report an issue at https://github.com/cask/cask/issues
The command "curl -fsSkL https://gist.github.com/rejeep/ebcd57c3af83b049833b/raw > travis.sh && source ./travis.sh" failed and exited with 1 during .

Your build has been stopped.
```

.travis.yml

@@ -6,7 +6,10 @@ env:
   - EVM_EMACS=emacs-26.3-travis
   - EVM_EMACS=emacs-27.1-travis
 before_install:
-  - curl -fsSkL https://gist.github.com/rejeep/ebcd57c3af83b049833b/raw > travis.sh && source ./travis.sh
+  - git clone https://github.com/cask/cask ~/.cask
+  - export PATH=$HOME/.cask/bin:$HOME/.evm/bin:$PATH
+  - git clone https://github.com/rejeep/evm.git /home/travis/.evm
+  - evm config path /tmp
   - evm install $EVM_EMACS --use --skip
   - cask
 script: