Add basic Logger spec

I recently added this class but didn't include any tests for it. This
commit adds tests for the two public methods.

spec/diffux_ci/logger_spec.rb

@@ -0,0 +1,42 @@
+require 'diffux_ci/logger'
+
+describe DiffuxCI::Logger do
+  let(:io)     { StringIO.new }
+  let(:logger) { described_class.new(io) }
+
+  describe '#log' do
+    subject { io.string }
+
+    it 'adds a newline' do
+      logger.log('Hi mom')
+      expect(subject).to eq "Hi mom\n"
+    end
+
+    context 'when disabling trailing newline' do
+      it 'does not add a newline' do
+        logger.log('Hi mom', false)
+        expect(subject).to eq 'Hi mom'
+      end
+    end
+  end
+
+  describe '#cyan' do
+    subject { logger.cyan('Hi mom') }
+
+    context 'when IO is a TTY' do
+      before do
+        allow(io).to receive(:tty?) { true }
+      end
+
+      it { should_not include '36' }
+    end
+
+    context 'when IO is not a TTY' do
+      before do
+        allow(io).to receive(:tty?) { false }
+      end
+
+      it { should include '36' }
+    end
+  end
+end