Use `make -j` to speed up Travis CI build.

Example  https://travis-ci.org/XhmikosR/libsass/builds/27940640

Before:
```
Job     Duration        Finished                    Compiler    ENV
380.1   3 min 26 sec    about 16 hours ago          gcc         AUTOTOOLS=yes
380.2   2 min           about 16 hours ago          gcc         AUTOTOOLS=no
380.3   2 min 34 sec    about 16 hours ago          clang       AUTOTOOLS=yes
380.4   1 min 16 sec    about 16 hours ago          clang       AUTOTOOLS=no
```

After:
```
Job     Duration        Finished                    Compiler    ENV
1.1     1 min           less than a minute ago      gcc         AUTOTOOLS=yes
1.2     24 sec          2 minutes ago               gcc         AUTOTOOLS=no
1.3     1 min 24 sec    less than a minute ago      clang       AUTOTOOLS=yes
1.4     17 sec          2 minutes ago               clang       AUTOTOOLS=no
```