Revert "use bash for the makefile shell"

This reverts commit 132a381.

The initial commit doesn't mention this, but the pull request has the
error messages this was avoiding [1]:

> make is using sh on Jenkins by default. This tweak sets the shell to
> bash to fix the following error on make check:
>
> 05:22:18 [1650506.967906] golang[6]: /bin/sh: 1: read: arg count
> 05:22:18 [1650507.054205] golang[6]: /bin/sh: 1: [: 2: unexpected operator

Instead of forcing Bash, we should fix those issues to stay
POSIX-compliant.

[1]: #870 (comment)

Author: wking

Makefile

@@ -2,7 +2,6 @@ export CGO_ENABLED:=0
 export GOARCH:=amd64
 export PATH:=$(PATH):$(PWD)
 
-SHELL:=$(shell which bash)
 LOCAL_OS:=$(shell uname | tr A-Z a-z)
 GOFILES:=$(shell find . -name '*.go' | grep -v -E '(./vendor)')
 LDFLAGS=-X github.com/kubernetes-incubator/bootkube/pkg/version.Version=$(shell $(CURDIR)/build/git-version.sh)