you just need to clean up the if statement:
<?php if ( has_post_thumbnail() ) { ?>
<div class="col-md-3">
<?php the_post_thumbnail(); ?>
</div>
<?php } ?>
here’s the whole bit cleaned up:
<?php query_posts('posts_per_page=1') ?>
<?php while ( have_posts() ) : the_post(); ?>
<div class="jumbotron">
<?php if ( has_post_thumbnail(); { )?>
<div class="col-md-3">
<?php the_post_thumbnail(); ?>
</div>
<?php } ?>
<div class="col-md-9">
<h2><a href="https://wordpress.stackexchange.com/questions/129375/<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
<p><?php the_excerpt(); ?></p>
<p class="text-muted">Posted on <?php the_time('jS F, Y'); ?></p>
</div>
</div>
<?php endwhile; wp_reset_query(); ?>