[Resolves Sceptre#921 and Sceptre#956] Remove Python 2.7 support and …

…fix build (Sceptre#959)

* [Resolves Sceptre#956] Fix sceptre build

Python 2.7 support was removed from `cloudreach/sceptre-circleci` docker image in commit
Sceptre/sceptre-circleci@975fa09
and tagged with https://github.com/Sceptre/sceptre-circleci/releases/tag/v0.5.0

We removed python 2.7 support from sceptre in PR Sceptre#922 however we neglected to update
cicleci to build with the updated docker image. This PR will update the docker image
to remove python 2.7 from the builds.

* update pip

* fix pip flag

* adjust updated version with PR sceptre-circleci/pull/1

.circleci/config.yml

@@ -10,7 +10,7 @@ executors:
 aliases:
   - &docs-job
     docker:
-      - image: cloudreach/sceptre-circleci:0.4
+      - image: cloudreach/sceptre-circleci:0.6.0
         environment:
           REPOSITORY_PATH: '/home/circleci/docs'
           DEPLOYMENT_GIT_SSH: 'git@github.com:Sceptre/sceptre.github.io.git'
@@ -32,7 +32,7 @@ aliases:
 jobs:
   build:
     docker:
-      - image: cloudreach/sceptre-circleci:0.4
+      - image: cloudreach/sceptre-circleci:0.6.0
     steps:
       - checkout
       - run:
@@ -69,7 +69,7 @@ jobs:
 
   lint-and-unit-tests:
     docker:
-      - image: cloudreach/sceptre-circleci:0.4
+      - image: cloudreach/sceptre-circleci:0.6.0
     steps:
       - attach_workspace:
           at: /home/circleci
@@ -139,7 +139,7 @@ jobs:
   integration-tests:
     parallelism: 2
     docker:
-      - image: cloudreach/sceptre-circleci:0.4
+      - image: cloudreach/sceptre-circleci:0.6.0
         environment:
           AWS_DEFAULT_REGION: eu-west-1
     steps:
@@ -182,7 +182,7 @@ jobs:
 
   deploy-pypi:
     docker:
-      - image: cloudreach/sceptre-circleci:0.4
+      - image: cloudreach/sceptre-circleci:0.6.0
     steps:
       - attach_workspace:
           at: /home/circleci