Fix wrong value type for HSTS environment variable

Seem like also environment variables are affected. This patch fixes that
as well.
Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

lib/config/environment.js

@@ -14,7 +14,7 @@ module.exports = {
   useSSL: toBooleanConfig(process.env.CMD_USESSL),
   hsts: {
     enable: toBooleanConfig(process.env.CMD_HSTS_ENABLE),
-    maxAgeSeconds: process.env.CMD_HSTS_MAX_AGE,
+    maxAgeSeconds: toIntegerConfig(process.env.CMD_HSTS_MAX_AGE),
     includeSubdomains: toBooleanConfig(process.env.CMD_HSTS_INCLUDE_SUBDOMAINS),
     preload: toBooleanConfig(process.env.CMD_HSTS_PRELOAD)
   },

lib/config/hackmdEnvironment.js

@@ -10,7 +10,7 @@ module.exports = {
   useSSL: toBooleanConfig(process.env.HMD_USESSL),
   hsts: {
     enable: toBooleanConfig(process.env.HMD_HSTS_ENABLE),
-    maxAgeSeconds: process.env.HMD_HSTS_MAX_AGE,
+    maxAgeSeconds: toIntegerConfig(process.env.HMD_HSTS_MAX_AGE),
     includeSubdomains: toBooleanConfig(process.env.HMD_HSTS_INCLUDE_SUBDOMAINS),
     preload: toBooleanConfig(process.env.HMD_HSTS_PRELOAD)
   },