Enable shellcheck and fix issues (#170)

.travis.yml

@@ -7,3 +7,5 @@ before_install:
 script:
   - bats test/kubectx.bats
   - bats test/kubens.bats
+  - shellcheck kubectx
+  - shellcheck kubens

kubens

@@ -66,7 +66,9 @@ escape_context_name() {
 }
 
 namespace_file() {
-  local ctx="$(escape_context_name "${1}")"
+  local ctx
+
+  ctx="$(escape_context_name "${1}")"
   echo "${KUBENS_DIR}/${ctx}"
 }