Add pandoc to needed dependencies

The pandoc command is needed when compiling with make to run the tests
```
mpereira@ubuntu:~/shellcheck$ make
: Conditionally compiling shellcheck
ghc -O9 --make shellcheck
[1 of 6] Compiling ShellCheck.AST   ( ShellCheck/AST.hs, ShellCheck/AST.o )
[2 of 6] Compiling ShellCheck.Data  ( ShellCheck/Data.hs, ShellCheck/Data.o )
[3 of 6] Compiling ShellCheck.Parser ( ShellCheck/Parser.hs, ShellCheck/Parser.o )
[4 of 6] Compiling ShellCheck.Analytics ( ShellCheck/Analytics.hs, ShellCheck/Analytics.o )
[5 of 6] Compiling ShellCheck.Simple ( ShellCheck/Simple.hs, ShellCheck/Simple.o )
[6 of 6] Compiling Main             ( shellcheck.hs, shellcheck.o )
Linking shellcheck ...
: Running unit tests
./test/runQuack && touch .tests
pandoc -s -t man shellcheck.1.md -o shellcheck.1
make: pandoc: Command not found
make: *** [shellcheck.1] Error 127
``` 
Once pandoc is installed:
```
mpereira@ubuntu:~/shellcheck$ sudo apt-get install pandoc
mpereira@ubuntu:~/shellcheck$ make
: Conditionally compiling shellcheck
ghc -O9 --make shellcheck
pandoc -s -t man shellcheck.1.md -o shellcheck.1
: Done
```