use Cocoapods 1.4.0 as latest versions fail validation

.travis.yml

@@ -12,7 +12,7 @@ matrix:
       osx_image: xcode10
       language: objective-c
       before_install:
-        - gem install cocoapods --version 1.6.0.beta.1 --no-document
+        - gem install cocoapods --version 1.4.0 --no-document
     - script:
         - swift package clean && swift build && swift test
       os: linux

Gemfile

@@ -0,0 +1,3 @@
+source "https://rubygems.org"
+
+gem "cocoapods", '=1.4.0'

Gemfile.lock

@@ -0,0 +1,76 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    CFPropertyList (3.0.0)
+    activesupport (4.2.11)
+      i18n (~> 0.7)
+      minitest (~> 5.1)
+      thread_safe (~> 0.3, >= 0.3.4)
+      tzinfo (~> 1.1)
+    atomos (0.1.3)
+    claide (1.0.2)
+    cocoapods (1.4.0)
+      activesupport (>= 4.0.2, < 5)
+      claide (>= 1.0.2, < 2.0)
+      cocoapods-core (= 1.4.0)
+      cocoapods-deintegrate (>= 1.0.2, < 2.0)
+      cocoapods-downloader (>= 1.1.3, < 2.0)
+      cocoapods-plugins (>= 1.0.0, < 2.0)
+      cocoapods-search (>= 1.0.0, < 2.0)
+      cocoapods-stats (>= 1.0.0, < 2.0)
+      cocoapods-trunk (>= 1.3.0, < 2.0)
+      cocoapods-try (>= 1.1.0, < 2.0)
+      colored2 (~> 3.1)
+      escape (~> 0.0.4)
+      fourflusher (~> 2.0.1)
+      gh_inspector (~> 1.0)
+      molinillo (~> 0.6.4)
+      nap (~> 1.0)
+      ruby-macho (~> 1.1)
+      xcodeproj (>= 1.5.4, < 2.0)
+    cocoapods-core (1.4.0)
+      activesupport (>= 4.0.2, < 6)
+      fuzzy_match (~> 2.0.4)
+      nap (~> 1.0)
+    cocoapods-deintegrate (1.0.2)
+    cocoapods-downloader (1.2.2)
+    cocoapods-plugins (1.0.0)
+      nap
+    cocoapods-search (1.0.0)
+    cocoapods-stats (1.0.0)
+    cocoapods-trunk (1.3.1)
+      nap (>= 0.8, < 2.0)
+      netrc (~> 0.11)
+    cocoapods-try (1.1.0)
+    colored2 (3.1.2)
+    concurrent-ruby (1.1.4)
+    escape (0.0.4)
+    fourflusher (2.0.1)
+    fuzzy_match (2.0.4)
+    gh_inspector (1.1.3)
+    i18n (0.9.5)
+      concurrent-ruby (~> 1.0)
+    minitest (5.11.3)
+    molinillo (0.6.6)
+    nanaimo (0.2.6)
+    nap (1.1.0)
+    netrc (0.11.0)
+    ruby-macho (1.3.1)
+    thread_safe (0.3.6)
+    tzinfo (1.2.5)
+      thread_safe (~> 0.1)
+    xcodeproj (1.7.0)
+      CFPropertyList (>= 2.3.3, < 4.0)
+      atomos (~> 0.1.3)
+      claide (>= 1.0.2, < 2.0)
+      colored2 (~> 3.1)
+      nanaimo (~> 0.2.6)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  cocoapods (= 1.4.0)
+
+BUNDLED WITH
+   1.16.5