Avoid building multi times

[sanemat]

If set multiple envs, e.g.

rvm:
  - 1.9.3
  - 2.0.0
  - 2.1

This run saddler 3times 😢

[sanemat]

If I'm in travis-ci, check TRAVIS_JOB_NUMBER like this.

#!/bin/bash
set -euv
if  [[ "${TRAVIS_JOB_NUMBER: -2}" == ".1" ]]; then
  # you want to do
fi

OR

#!/bin/bash
set -euv
if [[ "${TRAVIS_JOB_NUMBER}" =~ \.1$ ]]; then
  # you want to do
fi

• Actual example

[sanemat]

Need document.