Trying to fix MacOS Xcode 7.3 builds

Homebrew is not distributing binary bottles anymore for some (most?)
El Capitan packages, and thus we are being left with having to compile
them on our own, which takes longer than Travis would like. Let's thus
simply skip the installation of fftw (which pulls gcc and open-mpi) in
this flavour of Xcode, which will allow us to continue testing this
platform, though less intensively.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>

.travis/before_install.sh

@@ -37,19 +37,19 @@ then
 		brew update
 	fi
 
-	# The xcode 8.3 and 9.3 images need oclint to be uninstalled
-	# (see travis-ci issue #8826)
-	if [ "${XCODE}" = "8.3" -o "${XCODE}" = "9.3" ]
-	then
-		brew cask uninstall oclint
-	fi
-
 	# cxxtest pulls python@2, so we need to unlink
 	# the pre-installed python first
 	brew unlink python
 
 	# Minimal dependencies for testing
-	brew install gsl fftw cxxtest
+	brew install gsl cxxtest
+
+	# 7.3 (El Capitan) mostly doesn't have associated bottles anymore,
+	# and fftw is pulling both gcc and open-mpi, which take way too long
+	# to compile; let's simply skip this one
+	if [ "${XCODE}" != "7.3" ]; then
+		brew install fftw
+	fi
 	return
 fi