Move API key initialization outside the constructor (#150)

Because the API key utilizes the store scope during retrieval, doing so in the constructor fails if Magento hasn't been installed yet.  This prevents installing TaxJar at the same time as Magento.

Model/Transaction/Backfill.php

@@ -132,7 +132,6 @@ public function __construct(
         $this->filterGroupBuilder = $filterGroupBuilder;
         $this->searchCriteriaBuilder = $searchCriteriaBuilder;
         $this->taxjarConfig = $taxjarConfig;
-        $this->apiKey = $this->taxjarConfig->getApiKey();
     }
 
     /**
@@ -146,6 +145,8 @@ public function start(
     ) {
         // @codingStandardsIgnoreEnd
 
+        $this->apiKey = $this->taxjarConfig->getApiKey();
+
         if (!$this->apiKey) {
             throw new LocalizedException(__('Could not sync transactions with TaxJar. Please make sure you have an API key.'));
         }

Observer/ImportCategories.php

@@ -99,7 +99,6 @@ public function __construct(
         $this->categoryFactory = $categoryFactory;
         $this->categoryResourceModel = $categoryResourceModel;
         $this->taxjarConfig = $taxjarConfig;
-        $this->apiKey = $this->taxjarConfig->getApiKey();
     }
 
     /**
@@ -110,6 +109,8 @@ public function __construct(
     // @codingStandardsIgnoreStart
     public function execute(Observer $observer)
     {
+        $this->apiKey = $this->taxjarConfig->getApiKey();
+
         // @codingStandardsIgnoreEnd
         if ($this->apiKey) {
             $this->client = $this->clientFactory->create();