change python-pybabel to python-babel in package requirements

Closes #1147

dev/install/installation.html

 <h2><a class="toc-backref" href="#id4">Basic installation</a><a class="headerlink" href="#basic-installation" title="Permalink to this headline">¶</a></h2>
 <p>Step by step installation for Debian/Ubuntu with virtualenv. For Ubuntu, be sure to have enable universe repository.</p>
 <p>Install packages:</p>
-<div class="code sh highlight-default"><div class="highlight"><pre><span></span><span class="n">sudo</span> <span class="n">apt</span><span class="o">-</span><span class="n">get</span> <span class="n">install</span> <span class="n">git</span> <span class="n">build</span><span class="o">-</span><span class="n">essential</span> <span class="n">libxslt</span><span class="o">-</span><span class="n">dev</span> <span class="n">python</span><span class="o">-</span><span class="n">dev</span> <span class="n">python</span><span class="o">-</span><span class="n">virtualenv</span> <span class="n">python</span><span class="o">-</span><span class="n">pybabel</span> <span class="n">zlib1g</span><span class="o">-</span><span class="n">dev</span> <span class="n">libffi</span><span class="o">-</span><span class="n">dev</span> <span class="n">libssl</span><span class="o">-</span><span class="n">dev</span>
+<div class="code sh highlight-default"><div class="highlight"><pre><span></span><span class="n">sudo</span> <span class="n">apt</span><span class="o">-</span><span class="n">get</span> <span class="n">install</span> <span class="n">git</span> <span class="n">build</span><span class="o">-</span><span class="n">essential</span> <span class="n">libxslt</span><span class="o">-</span><span class="n">dev</span> <span class="n">python</span><span class="o">-</span><span class="n">dev</span> <span class="n">python</span><span class="o">-</span><span class="n">virtualenv</span> <span class="n">python</span><span class="o">-</span><span class="n">babel</span> <span class="n">zlib1g</span><span class="o">-</span><span class="n">dev</span> <span class="n">libffi</span><span class="o">-</span><span class="n">dev</span> <span class="n">libssl</span><span class="o">-</span><span class="n">dev</span>
 </pre></div>
 </div>
 <p>Install searx:</p>

docs/dev/install/installation.rst

 
 .. code:: sh
 
-    sudo apt-get install git build-essential libxslt-dev python-dev python-virtualenv python-pybabel zlib1g-dev libffi-dev libssl-dev
+    sudo apt-get install git build-essential libxslt-dev python-dev python-virtualenv python-babel zlib1g-dev libffi-dev libssl-dev
 
 Install searx: