Removes executable flag from LICENCE.

No need for the `LICENCE` file to have the executable flag set. Now it's consistent with the other text files in the root directory.

Before: `ls -la .`
```
...
-rw-r--r--   1 jbnicolai  staff    84 Jul  6 23:42 .travis.yml
-rw-r--r--   1 jbnicolai  staff  4656 Jul  6 23:42 CHANGELOG.md
-rw-r--r--   1 jbnicolai  staff  3705 Jul  6 23:42 CONTRIBUTING.md
-rwxr-xr-x   1 jbnicolai  staff  1078 Jul  6 23:42 LICENSE
-rw-r--r--   1 jbnicolai  staff  3327 Jul  6 23:42 README.md
...
```

After: `ls -la .`
```
...
-rw-r--r--   1 jbnicolai  staff    84 Jul  6 23:42 .travis.yml
-rw-r--r--   1 jbnicolai  staff  4656 Jul  6 23:42 CHANGELOG.md
-rw-r--r--   1 jbnicolai  staff  3705 Jul  6 23:42 CONTRIBUTING.md
-rw-r--r--   1 jbnicolai  staff  1078 Jul  6 23:42 LICENSE
-rw-r--r--   1 jbnicolai  staff  3327 Jul  6 23:42 README.md
...
```