requirements.txt: remove unnecessary version pins that conflict with piksi_tools
-boto==2.42.0
construct==2.5.2
-httpretty
requests>=2.8.1
-futures==2.2.0
requests-futures>=0.9.5
+futures>=2.2.0
+boto
+httpretty
pyftdi
pylibftdi
-pyserial==2.7
-pyyaml==3.11
+pyserial
+pyyaml
