Add .sc as a SuperCollider file extension

This pull request adds `.sc` as a SuperCollider file extension as requested at #1091.
It is the standard one for SuperCollider class files.
This extension is already used by Scala.

I tested it on several Scala and SuperCollider repositories.
It's not perfect but I guess it's better than before since it wasn't recognized at all.
- [scala-for-the-impatient](https://github.com/bodiam/scala-for-the-impatient.git) - Scala

|Without heuristics|With heuristics|
|---|---|
|95.41% Scala<br/>4.59% SuperCollider|99.51% Scala<br/>0.49% SuperCollider|

- [scala4impatientasks](https://github.com/adsinc/scala4impatientasks.git) - Scala

|Without heuristics|With heuristics|
|---|---|
98.59% Scala<br/>1.41% SuperCollider|99.96% Scala<br/>0.04% SuperCollider|

- [scala-quiz](https://github.com/codingnirvana/scala-quiz.git) - Scala

|Without heuristics|With heuristics|
|---|---|
91.09% Scala<br/>8.91% Java|91.09% Scala<br/>8.91% Java|

- [Scala-learning](https://github.com/iBuddha/Scala-learning.git) - Scala

|Without heuristics|With heuristics|
|---|---|
91.43% Scala<br/>8.27% SuperCollider<br/>0.30% Java|98.07% Scala<br/>1.63% SuperCollider<br/>0.30% Java|

- [hack_listener](https://github.com/audiohackersalon/hack_listener.git) - SuperCollider

|Without heuristics|With heuristics|
|---|---|
100.00% SuperCollider|100.00% SuperCollider|

- [SuperCute](https://github.com/danstowell/SuperCute.git) - SuperCollider

|Without heuristics|With heuristics|
|---|---|
53.73% C++<br/>23.45% SuperCollider<br/>13.55% C<br/>2.03% Python<br/>1.82% Emacs Lisp<br/>1.75% Objective-C++<br/>1.57% Objective-C<br/>0.65% Scala<br/>0.58% Perl<br/>0.46% VimL<br/>0.22% Ruby<br/>0.10% Shell<br/>0.06% Rebol<br/>0.03% R|53.73% C++<br/>24.10% SuperCollider<br/>13.55% C<br/>2.03% Python<br/>1.82% Emacs Lisp<br/>1.75% Objective-C++<br/>1.57% Objective-C<br/>0.58% Perl<br/>0.46% VimL<br/>0.22% Ruby<br/>0.10% Shell<br/>0.06% Rebol<br/>0.03% R|

- [Thr44-SC-quark](https://github.com/Thr44/Thr44-SC-quark.git) - SuperCollider

|Without heuristics|With heuristics|
|---|---|
100.00% SuperCollider|100.00% SuperCollider|

EDIT: The [Travis build](https://travis-ci.org/github/linguist/builds/38716628) is passing.