Install GnuTLS in Travis builds

flycheck/emacs-travis#22
https://github.com/flycheck/emacs-travis/blob/f72e74ac6c3c6bf9042c72deee22f2a3a3e596cf/emacs-travis.mk#L105

Use HTTPS when fetching from ELPA/MELPA

Makefile

@@ -1,4 +1,4 @@
-deps:
+deps: install_gnutls
 	emacs \
 		-batch \
 		-l tests/setup/init.el \
@@ -24,4 +24,22 @@ test:
 build:
 	@:
 
-.PHONY: deps test build
+install_gnutls:
+ifeq ($(TRAVIS_OS_NAME),osx)
+#	GnuTLS can be upgraded with Homebrew instead of apt-get (which obviously does not exist on macOS)
+	@echo "Upgrade GnuTLS 3"
+	@if brew outdated --quiet | grep --quiet "gnutls"; then brew upgrade gnutls; fi
+else
+	@echo "Install GnuTLS 3"
+	@sudo apt-get -qq update
+	@sudo apt-get install -y build-essential nettle-dev libgmp-dev
+	@wget ftp://ftp.gnutls.org/gcrypt/gnutls/v3.1/gnutls-3.1.23.tar.xz
+	@tar -xf gnutls-3.1.23.tar.xz
+	@cd gnutls-3.1.23 \
+	  && ./configure $(SILENT) \
+	  && make -j$(MAKE_JOBS) $(SILENT) \
+	  && sudo make install $(SILENT) \
+	  && sudo ln -s /usr/local/lib/libgnutls.so.28 /usr/lib/libgnutls.so.28
+endif
+
+.PHONY: deps test build install_gnutls

tests/setup/install-deps.el

@@ -11,9 +11,9 @@
 (defun xenops-dependencies-setup-package-repositories ()
   (setq package-archives
         (list (if melpa-stable
-                  '("melpa-stable" . "http://stable.melpa.org/packages/")
-                '("melpa" . "http://melpa.org/packages/"))
-              '("gnu" . "http://elpa.gnu.org/packages/")))
+                  '("melpa-stable" . "https://stable.melpa.org/packages/")
+                '("melpa" . "https://melpa.org/packages/"))
+              '("gnu" . "https://elpa.gnu.org/packages/")))
   (package-refresh-contents)
   t)