Stack 1.4.0 - libgmp Needs to Be Manually Installed in Ubuntu (Travis CI)

When running stack 1.4.0 in Ubuntu (Travis CI), I got an error about not being able to find **libgmp**. The workaround is to add this as a package to pre-install in `.travis.yml`, but I did not need to do this for previous versions of stack. The error is below.

```
$ stack --version
Version 1.4.0, Git revision e714f1dd3fade19496d91bd6a017e435a96a6bcd (4640 commits) x86_64 hpack-0.17.0
```

```
Downloaded ghc-8.0.1.
Unpacking GHC into /home/travis/.stack/programs/x86_64-linux/ghc-8.0.1.temp/ ...
Configuring GHC ...
Installing GHC ...
Installed GHC.
The GHC located at /home/travis/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc failed to compile a sanity check. Please see:
http://docs.haskellstack.org/en/stable/install_and_upgrade/
for more information. Exception was:
Running /home/travis/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc /tmp/stack-sanity-check2960/Main.hs -no-user-package-db in directory /tmp/stack-sanity-check2960/ exited with ExitFailure 1
[1 of 1] Compiling Main ( /tmp/stack-sanity-check2960/Main.hs, /tmp/stack-sanity-check2960/Main.o )
Linking /tmp/stack-sanity-check2960/Main ...
/usr/bin/ld: cannot find -lgmp
collect2: ld returned 1 exit status
'gcc' failed in phase 'Linker'. (Exit code: 1)
```