Update .travis.yml

[RuiSiang]

Changed dependency, as proposed in PR #84

[coveralls]

Coverage remained the same at 93.103% when pulling b5f738a on RuiSiang:patch-1 into c177b65 on lukechilds:master.

[lukechilds]

Thanks for taking the time to submit this. Why is this change needed?

[RuiSiang]

After this PR, try merging PR #84 again.

libjpeg-dev and libjpeg8-dev are for different libraries. libjpeg-dev is for libjpeg-turbo, while libjpeg8-dev is for the original libjpeg. Canvas changed dependencies a while back. That's why tests failed starting from PR #64(the PR prompt travis to compile canvas while missing a dependency).

Another reason is that libjpeg8-dev is not available on Debian Jessie, might be another reason to change it

Hope that clarifies.

[lukechilds]

After this PR, try merging PR #84 again.

Merging this won't help #84, see #90 (comment).

Canvas changed dependencies a while back. That's why tests failed starting from PR #64(the PR prompt travis to compile canvas while missing a dependency)

But tests are passing in master with libjpeg8-dev and canvas@^2.6.1.